1. Field
The disclosure relates generally to a quantum key distribution system and, more particularly, to a method and system for transmitting optical clock signals and quantum key signals on a single optical channel in a free space quantum key distribution system, and for reducing quantum bit error rate in a free space quantum key distribution system.
2. Background
Quantum Key Distribution (QKD), which was first recognized in the early 1990s as a means to produce a random bit stream for encryption, has recently seen increased interest from both military and civil spheres as providing information assurance, creating a secure communications environment. In a QKD system, lasers are used to generate quantum mechanically coupled photon states. The nature of this coupling allows the photons to simultaneously exist in orthogonal modes. These orthogonal modes are typically manifested in optical polarization, with orthogonal modes being vertical and horizontal linear or left and right circular. The random nature of the quantum mechanical process will randomly, with equal probability, cause measurement of one orthogonal state or the other. With an assignment of bit value “0” or “1” to one polarization state measurement, a truly random bit stream will be generated. The stream may be referred to as a “key”, according to present encryption architectures. As this key is represented by a quantum state, it is referred to as “quantum signals” or “quantum key signals.”
Essential to the security of the quantum key transmission is that a quantum state cannot be cloned, and that a quantum state cannot be identified without being disturbed. In a typical QKD protocol, the transmitter and receiver randomly select their basis set (for example, vertical vs. horizontal polarization) before each photon is detected. After transmission of the key, the receiver and transmitter compare basis sets, to assess whether a given measurement was made with orthogonal detection. Statistically, half of the settings will be wrong and the corresponding signals discarded. Half of the transmitter/receiver mode will be correct, and the transmitter/receiver will keep that bit. After some predetermined length of key has been collected, depending on the application in question, the transmitter and the receiver will have the same encryption key, which is then used for transmission of secure data.
QKD in free space (FS) may become a crucial technique for a secure space-air-ground information network. In a free space QKD system, a transmitter sends keys via single photons to a receiver through free space to be detected by a single photon detector (SPD) at a receiver. As in any digital signal transmission system, clock recovery and synchronization between the transmitter and the receiver are required for proper operation.
Currently, clock recovery and synchronization in a free space QKD system are realized by establishing a second optical path, sometimes called a “classical channel”, to transmit optical pulses. This classical channel needs to synchronize with the quantum channel that carries the quantum key signals in order to achieve synchronization between the transmitter and the receiver.
It is known in a conventional optical communications mechanism, including a transmitter and receiver, that optical transmission modules be included to transmit clock signals. The classical and quantum channel have fundamentally distinct tasks. Therefore, there must be physical mechanisms to assure the distinction between photons in each channel. At the same time, the transmitter and receiver may be physically co-located, requiring co-incident or nearly co-incident physical optical paths. With synchronization pulses at the nanojoule level, there is an 8 to 9 order of magnitude difference between synchronization and quantum channel signal strengths. One way to separate the signals is wavelength multiplexing, with the two channels transmitting at different optical wavelengths.
Since the classical channel and the quantum channel may use different wavelengths, it is also known to transmit both the classical signals and the quantum signals via the same optical path. The two signals can be combined and separated by wavelength division techniques, with combining or separation accomplished with techniques including but not limited to prismatic, grating, or interferometric hardware.
The quantum channel, however, is very sensitive to noise inasmuch as single photon detectors (SPDs) must be tuned to be extremely sensitive in order to be able to respond to single photons. Stray photons from the synchronization or other external source can easily obscure the quantum channel signal. As a result, substantial care is required to effectively isolate the SPD from excitation by the classical signals. This results in the need for a QKD system that has relatively complex hardware, increased environmental sensitivity and that is of substantial size and weight.
Also, in a free space QKD system, quantum states are prone to be lost due to factors such as transmission losses, de-coherence, and interruptions from a potential eavesdropper. Accordingly, an important criterion in evaluating a free space QKD system is its quantum bit error rate (QBER).
In order to achieve an acceptable QBER, the photon source in current QKD systems includes a cooled laser diode or a VCSEL (Vertical Cavity Surface Emitting Laser) driven by a customized laser driver to produce sharp laser pulses (<1 ns), in conjunction with a pseudo random number generator to produce arbitrary quantum keys. Such a photon source, however, is fairly complicated and contributes to providing a relatively complex QKD system. A photon source that is composed of only two, relatively low cost, commercially available, off-the-shelf parts, specifically, a single mode VCSEL and a data generator, would be simple and modular in construction and provide a QKD system that is robust and of reduced size and weight.
In current QKD systems, in order to maximize the probability that a received quantum key signal accurately corresponds to a transmitted quantum key signal and to minimize QBER, the photon source in the transmitter and the SPD in the receiver are synchronized such that the SPD detects the received quantum key signal at a timing that corresponds to the timing of the transmitted quantum key signal. A pulse produced by a single mode VCSEL, however, may have a relatively long tail (e.g., greater than about 8 ns). The length of the tail, in fact, may be longer than the sampling period of the SPD with the result being that the SPD may detect the photon emitted at the tail and produce an erroneous output signal. Such detection errors can result in the QKD system having an unacceptably high QBER.
There is, accordingly, a need for improved mechanisms for achieving clock recovery and synchronization and for reducing quantum bit error rate in a free space quantum key distribution system, such as a free space quantum key distribution system utilizing a single mode VCSEL as a photon source.